Thoughts on trying the Van Halen Variac trick with a rectifier

[Jorn218]

I’ve been listening to the Tone Talk podcast as of late and in one episode Dave Friedman explains exactly how EVH ran his variac wig his amps.

What Van Halen did, if you did not know was to take a variac and drop incoming voltage to the amp to around 90VAC instead of the 110-115VAC that was standard at the time. That alone would not get his tone from his Marshall Super Lead amplifiers. The key is to rebias the amplifier to spec after reducing the incoming electricity to the amplifier. This apparently reduces the headroom of the amplifier allowing for the power section to break up pleasantly. At least for Marshall Super Lead amplifiers.

Thinking about modifications for my dual rectifier and installing the adjustable bias mod to my amp already (not to bias my amp hot or cold, but to widen my ability to use pretty much whatever tubes I can obtain), as well as planning the effects loop mod, I’m curious to how running the amplifier with reduced supply voltage, then rebiasing the amp would sound?

The Bold/Spongy switch alone would not work because the amps drop in voltage doesn’t compensate for the tubes not having proper operating voltage.

Has anyone here tried anything like this? I know the amp shuts off completely until around 50VAC. But I was going to try down to about 90-100VAC at the lowest. Of course I’m not planning on permenantly running the amp this way but think it would be fairly interesting to see what happens with this circuit.

 

[tbonesullivan]

Hmm.. Maybe make a second switch that changes the tube bias when the spongy is engaged? I searched online and read about someone who has a Suhr SL68 with a 120/90v power transformer, and two bias pots, one to go with each power mode, which switch automatically. Not sure how that would be accomplished.

 

[bandit2013]

I read somewhere that was just BS. A way to throw off others on how to discover your tone and or sound if you wanted to be unique about it.

Thought the that is what the spongy switch did, drops the AC voltage on the primary side of the PT based on a change winding as it has a tapped primary.

Not sure that is a wise idea. Do what you want it is your gear. You can always find a replacement or repair if it suffers any damage. I have variacs at work, but not something I would be willing to try.

 

[domct203]

The Bold/Spongy switch alone would not work because the amps drop in voltage doesn’t compensate for the tubes not having proper operating voltage.

Spongy drops the voltage to the entire amp, exactly like a variac, it’s just at a fixed value.

Spongy is its own tap on the primary side of the power transformer, all voltage on the secondary side will be reduced.

Dom

 

[msi]

I would advise you to not do that. The rectifier's switching circuit uses a LM3914 to select channels. It is a chip that enables the channel at a specific voltage. If you change the voltage of the amp too much the switching might stop working. Also it is not good for the tubes to run their heaters under voltage.

 

[Mile High Effects]

I have installed an adjustable bias (using the 25K return send pot since I also performed the series loop mod and I currently use the same Variac he used but I only drop to around 115VAC so that the heaters are seeing 6.3v for power and preamp tubes and 5v for the rectifiers. Otherwise, they were up near 6.7V or something like that.

All rectifier and bold spongy settings work as they should.

 

[tbonesullivan]

Spongy drops the voltage to the entire amp, exactly like a variac, it’s just at a fixed value.

Spongy is its own tap on the primary side of the power transformer, all voltage on the secondary side will be reduced.

Dom

True, but I think he is talking about how when you drop the voltage, the power tubes will be biased colder as well, as they will have reduced plate voltage. I don't know if the current increases to compensate, or whether they just end up at a lower bias point. Mesa biases their amplifiers fairly cold by some standards, and dropping the voltage a lot could put them into the crossover distortion range.

 

[79fwlthree]

Just a thought. Since Power=voltage x current, then if you drop the voltage the current the amp draws is going to go up affecting all the things the transformer does and wasn't designed for. So, this is a modern amp designed for modern grid voltage. Check your AC outlets and you may be surprised to find that you are at 120 volts to 125 volts as I am here in south Florida. I only use my variac on amps that were designed for 110 volts; e.g. my vintage stuff from 60's (Supro, Silvertone, Princeton Reverb '65 (a real one). Mesa has a "tweed" switch on most of their amps that drops the voltage to a safe and calculated level.

 

[Mile High Effects]

If your rectifier is from the 90’s like mine, it will actually run more in-spec if you run it at ~117VAC instead of modern voltages (122-126V average).

The amp runs hotter at 120-125VAC. Like I mentioned, I went off the heater voltages and made sure they’re running at their intended voltages of 6.3V and 5V. Also going off of the Bias voltage tap as well.

I run all my amps on variacs to ensure proper heater voltages. It’s usually around 110-115V depending on the amp. You can even run it a little cooler to ensure a more extended life of the cathodes in the tubes. But I wouldn’t run it at 90VAC. You can get what is called cathode poisoning and actually shorten the tube life. Sometimes, dramatically.

 

[ezersky]

Get a Brown Box…quieter too…

 

[domct203]

True, but I think he is talking about how when you drop the voltage, the power tubes will be biased colder as well, as they will have reduced plate voltage. I don't know if the current increases to compensate, or whether they just end up at a lower bias point. Mesa biases their amplifiers fairly cold by some standards, and dropping the voltage a lot could put them into the crossover distortion range.

Spongy mode reduces overall voltage to the amp exactly the same as an external variac.

Sure, you can ‘adjust’ bias on the output tubes to compensate for the reduced voltage, but bottom line is still plate dissipation. As plate voltage drops plate current needs to increase to reach your target dissipation in watts. Watts = Amps x Volts.

Dom

 

[jpistolas]

I always assumed the idea was to run the variac down to what the older amps/transformers were used to seeing. Used to be 110v or less. Now it's 115v & sometimes a tad more. The studio was probably wired up to more modern spec. even back then.

 

[Mile High Effects]

Using it as a tool to run my amps to spec is really all I use mine for. Maybe a tad under-spec on my Marshall JTM45/100 at 110v. 560 volts on the screens is hella hard on modern KT66’s.

 

[Jorn218]

Spongy drops the voltage to the entire amp, exactly like a variac, it’s just at a fixed value.

Spongy is its own tap on the primary side of the power transformer, all voltage on the secondary side will be reduced.

Dom

Dom, the Spongy switch also takes in consideration for lowered power to the heaters as well? I was under the assumption that this switch just drops the voltage globally making the whole amp run colder and that even the heaters are running with lowered voltage ie the bias is even colder than when not on spongy.

 

[Jorn218]

Using it as a tool to run my amps to spec is really all I use mine for. Maybe a tad under-spec on my Marshall JTM45/100 at 110v. 560 volts on the screens is hella hard on modern KT66’s.

I can imagine. For a time I had a JCM800 clone built using an old Bradshaw circuit mod I got from a guy on Metro forum. I used a Heyboer PT that was supplying around that as B+. Essentially all the circuit mod was to push the amp just barely on the safe side of running off the rails. The only modern tubes that wouldn’t only last a day weee Jj KT88s. I was able to get Pantera levels of distortion from the amp with the preamp control about 2:00. Any more and the amp would create harsh almost digital sounding distortion. Fun amp but almost unusable in a band context.

 

[Jorn218]

I always assumed the idea was to run the variac down to what the older amps/transformers were used to seeing. Used to be 110v or less. Now it's 115v & sometimes a tad more. The studio was probably wired up to more modern spec. even back then.

The story was that since Ed ran everything wide open all the time, he would blow tubes if the amp was a wall voltage which maxed out in the late 70s around 115-120. To keep him from doing this, he ran it around 90VAVC then rebiased the amp at that voltage giving him the trademark tone.

The whole story takes place over every podcast where Evo is discussed. Especially the one with Egnator, Grover Jackson, the Peavey engineer who runs Amptweaker now, and the post death show. There are more, but those are the main ones that fill you in on how his tone was achieved. I know this is a Mesa board, but it’s still interesting information to learn. Especially once you find out that the amp was in fact bone stock and never modded as he claimed.

 

[Jorn218]

Spongy mode reduces overall voltage to the amp exactly the same as an external variac.

Sure, you can ‘adjust’ bias on the output tubes to compensate for the reduced voltage, but bottom line is still plate dissipation. As plate voltage drops plate current needs to increase to reach your target dissipation in watts. Watts = Amps x Volts.

Dom

I get that but you can still bias for dissipation at what you get on the plates. So for example your shooting for 60-70% dissipation at 100watts from wall voltage, well if you reduce the supply so your max wattage is now 55watts for example, you still bias as normal but have to use the voltage figure you get from the reduced supply voltage. (Using this method to achieve bias for example Set Bias ) I was just curious on how the amp (dual rectifier circuit)would sound. I know radically reducing supply voltage isn’t a factor since the amp won’t even turn on til around 53-55vAC as I know some load is needed to actually power the components to work.

Spongy drops supply voltage but to what percentage compared to full wall voltage? Also does the spongy section then compensate to raise the bias circuit to the same value the bold section runs at albeit reduced voltage? I’ve always heard that it just drops the voltage globally and the amp runs off the reduced voltage. No compensation is used within the circuit.

My thought was to drop the voltage, then turn the bias circuit up to normal (not spongy) milliamp/millivolt values, which would somewhat starve the amp in other areas would it not? I’m also not having radical experiments with supply voltage but more along the lines of reducing the supply voltage no more than to 85vac at maximum since current supply wall voltage is now hovering around 126-130VAC at least in my area.

 

[Jorn218]

Hmm.. Maybe make a second switch that changes the tube bias when the spongy is engaged? I searched online and read about someone who has a Suhr SL68 with a 120/90v power transformer, and two bias pots, one to go with each power mode, which switch automatically. Not sure how that would be accomplished.

Neither do I except I now installed a trim pot to dial in to correct bias as per stock spec.regardless of where I purchase tubes as sometimes you can request if they will work in a Mesa amp.

 

[Jorn218]

I read somewhere that was just BS. A way to throw off others on how to discover your tone and or sound if you wanted to be unique about it.

Thought the that is what the spongy switch did, drops the AC voltage on the primary side of the PT based on a change winding as it has a tapped primary.

Not sure that is a wise idea. Do what you want it is your gear. You can always find a replacement or repair if it suffers any damage. I have variacs at work, but not something I would be willing to try.

I’ve read the same. But my interest is not using Spongy but keep the amp on hold, lowering the voltage to the primary then biasing the amp to correct stock spec. You can’t lower the supply too much and still get enough electricity to be able to do this. I’m also not after Van Halen tone either. Just interested in how a dual rectifier would function and sound if similar process was applied to the amp.

 

[Jorn218]

Spongy drops the voltage to the entire amp, exactly like a variac, it’s just at a fixed value.

Spongy is its own tap on the primary side of the power transformer, all voltage on the secondary side will be reduced.

Dom

Yes but is tye bias circuit running at a reduced voltage reducing the bias to be even colder or does it run at normal values as if not reduced? That’s my line of thinking.